Polymer resins, such as polyethylene terephthalate (PET), are widely used in the packaging industry. PET is a linear, thermoplastic polyester resin. The myriad of advantages of PET include toughness, clarity, good barrier properties, light weight, design flexibility, chemical resistance, and good shelf-life performance. Furthermore, PET is environmentally friendly because it can often be recycled. These characteristics of PET make it a popular material in the manufacturing of containers, for example, beverage bottles.
There are a variety of production methodologies to produce PET containers. For example, injection stretch blow molding is commonly used to make PET bottles. Of the various methodologies, one-piece PET containers having an integrated handle are commonly formed using extrusion blow molding (EBM). The EBM process includes extruding a polymer resin in a softened state through an annular die to form a molten hollow tube (also called a “parison”). The molten parison is placed in a hollow blow mold having a cavity corresponding to the desired shape of the container being formed. Air is injected to inflate the parison against the interior walls of the blow mold. Upon contact with the walls, the parison cools rapidly and assumes the shape of the mold.
To make PET suitable for EBM, PET manufacturers have developed special grades of PET also referred to as extrusion grade PET or “EPET.” Typically, EPET is high molecular weight PET having an inherent viscosity (“I.V.”) of 1.0 dl/g or greater as measured by solution viscosity.
One EBM process used to manufacture EPET containers includes a vertical rotary blow molding machine, which can have an output of, for example, over 100 bottles per minute depending on the number of cavities and molds. Vertical rotary blow molding machines index circumferentially spaced mold halves on a wheel in steps around a horizontal axis. The mold halves each capture a vertical, continuously growing parison at an extrusion station. In one type of machine, for example, the flow head extruding the parison moves up and away from the mold halves after the mold halves close to capture the parison. The parison is severed adjacent the top of the mold halves, the mold halves are moved away from the extrusion station, and a top blow pin is moved into the end of the captured parison at the top of the mold halves to seal the mold cavity and blow the parison. Subsequently, the flow head and dependent parison are lowered back to the initial position so that the new parison is in position to be captured by the next pair of mold halves. The blown parison cools as the mold halves are rotated around the machine, following which the mold halves open at an ejection station and the finished article, commonly a container, is ejected from between the mold halves. In operation, the rotary wheel can produce at least 110 bottles per minute for a 22 cavity mold machine (22 molds) rotating at a rate of 5 RPM.
In EBM processes, high clamp forces are often required and deflection of the mold halves may occur. As a result, there may also be increased part wear and subsequent mold misalignment. Accordingly, there is a need in the art for a system for extrusion blow molding EPET containers that reduces deflection and operates at high clamp forces.